1. Field of the Invention
The present invention relates to a structure of a semiconductor device including a thin film transistor (TFT) formed on a substrate and a method of manufacturing the same.
2. Description of the Related Art
A liquid crystal display (LCD) has attracted considerable attention as a picture display device that can replace with a CRT, because of such features as being thin in shape, light weight, and low consumption of electric power.
Among various kinds of liquid crystal displays, there is a TFT liquid crystal display (TFT-LCD). This is an active matrix driving type liquid crystal display in which a thin film transistor (TFT) is used as a switching element of a pixel.
In the active matrix type liquid crystal display, a number of TFTs disposed for each pixel are formed on an insulating substrate so that an active matrix circuit is constituted. In recent years, there is also proposed a display in which a driver circuit for driving an active matrix circuit is also formed on an insulating substrate. This driver circuit is also constituted by a number of elements such as TFTs.
Like this, a number of TFTs are formed on an insulating substrate in an active matrix type liquid crystal display, it is weak against static electricity. Charges of the static electricity are produced when the liquid crystal display is rubbed with a person or a thing, and that an insulating substrate takes the electrical charges.
The TFT, especially, is apt to be damaged by the retained charges of the static electricity. This is because its gate insulating film is as thin as 50 to 200 nm, so that if a high voltage is applied to the gate insulating film by the charges of the static electricity, breakdown is liable to be occurred. Moreover, since a large current flows through an active layer of the TFT by the charges of the static electricity, the active layer is degraded. In a worst case, the active layer is peeled off.
If the TFT is damaged, the pixel of the liquid crystal display keeps an on-state or off-state. As a result, a defect occurs in the display of the liquid crystal display.
Such electrostatic breakdown can occur not only during the use of the liquid crystal display but also during the manufacture of the liquid crystal display. During the manufacture, the static electricity is liable to be produced through contact with a person or a device.